1. Field of the Invention
This invention relates to optical devices and, more particularly, to such devices as may be employed in exercising or training the eye muscles.
2. Description of the Prior Art
In human beings, it is the function of using two eyes together that keeps the optical axes straight in a normal person. Without normal muscular control, for example in the case where one eye becomes blind, that eye may tend to revert to an anatomical position of rest in line with the axis of the orbit, which causes it to appear to be diverging. The visual axes can remain straight only if each eye has reasonably good vision, the ocular muscles can move the eyes in the required direction of gaze, and the complex neuromuscular reflexes required to coordinate the movements of the two eyes are intact. Failure to maintain the visual axes parallel may therefore result from a visual defect in one or both eyes, a muscular defect resulting in loss of normal movement of the eye, or a defect in the central nervous system involving the coordinating nervous pathways.
A true squint is a condition in which the visual axes are no longer parallel. Clinically, squints are divided into concomitant and paralytic types. In the first type, the abnormal angle between the visual axes remains constant in all positions of gaze, whereas in the latter type, the angle of squint varies with the direction of gaze. The commonest type of squint is the convergent concomitant type seen in small children who are consistently or intermittently cross-eyed.
To view objects close by, the visual axes must converge, so that both eyes can view the same object, and the focus of the eye must be adjusted for near vision. The link between convergence of the eyes and focusing, or accommodation, is very strong and normally the two actions work in harmony. Most small children are far-sighted, which means that in order to see clearly close to, they have to exert an extra amount of accommodative effort. As accommodation and convergence are closely linked, the extra effort of accommodation tends to produce an overconvergence; but, provided that the visual acuity of each eye is normal and the motor control of the eyes is normal, this tendency is controlled. However, if the vision of one eye is reduced, for example by disease or an error of refraction, binocular vision breaks down and overconvergence occurs. Sometimes convergent concomitance is precipitated in young children by a systemic disease such as measles, or there may be a family history of squint.
Once parallelism of the visual axes is lost, the image of objects no longer lies on a familar area or retina, and instead of the images from the two eyes being fused into one, two images are perceived. This condition of double vision, or diplopia, is intolerable to the child, who reacts by "suppressing" the image from the squinting eye. If the suppression is allowed to continue, the central vision of the affected eye drops rapidly to a low level, so that even if the original disturbances that started the squint is corrected, this loss of vision, or amblyopia, of the squinting eye will prevent the restoration of normal binocular vision and thus perpetuate the squint. The longer the suppression is allowed to continue, the less likely is the child to regain normal vision in the squinting eye. Covering the good eye generally encourages the recovery of the suppressed vision, but to be effective it must be started as soon as the squint is noticed. Retraining of the binocular reflexes can be aided by special exercises.
In contrast, many people do not have the perfect balance of accommodation and convergence that enables the movements of the two eyes to bring the visual axes to the point of focus for all distances of vision. In such cases, the eyes may tend to converge or diverge too much for a given distance, a condition known as heterophoria.
While special mechanism having adjustable lenses has been developed for use by optometrists and other specialists, these devices are usually costly and relatively cumbersome and are not suitable for use in the home by the individual patient. Home training and exercise aids for correcting squint are generally limited to an opaque lens or sheet for covering one eye or to exercises, such as concentrating one's vision on one's fingertip while moving it toward or away from one's face. What is needed is a simple, lightweight, portable, inexpensive device which includes a lens system comparable to those employed for the purpose in a specialist's office but which is feasible for purchase and use by the individual patient in his habitual environment, such as the home or office.